\documentclass[a4paper, 12pt]{article}

\begin{document}

if $f(x) = 3x + 7$ and $g(x) = x + 4$ then
\[ f(x) + g(x) = 4x + 11 \]
and
\[ f(x)g(x) = 3x^2 + 19x + 28. \]

\[ u' + v'' \]

\end{document}
